The invention relates in general to the reduction of noise in electrical circuits, and in particular to the reduction of switching noise associated with a noise shaper that is functioning within an electrical circuit.
In the processing of data by a data processor, such as for example a digital/analog converter or a pulse width modulator, it may be advantageous to use a noise shaper to improve the signal-to-noise ratio in a desired frequency range (e.g., an audible range) with regard to any background noise that may be present. In this process, the background noise in a lower frequency range may be reduced by shifting the signal energy components to higher frequencies, which are not needed and may not be audible. The noise shaper may be used for example in systems in which amplitude quantization is carried out.
FIG. 4 illustrates a prior art circuit 10 having a noise shaper 12. A signal comprising input data of a data sequence may be provided on a line 14 to an adder 16, the resultant sum is output on a line 20 to a data processor 18. The data processed in the data processor 18 may be provided on a line 22 at an output. The data input to the data processor 18 on the line 20 may be subtracted from the data processor output on the line 22 by a subtractor 24. The result of the subtraction forms an error signal, which may be provided as an input signal on a line 26 to the noise shaper 12. The output signal from the noise shaper 12 on a line 28 may be added to or subtracted from the input data on the line 14 by the adder 16. In the case of a first-order noise filter, the error signal on the line 26 may be provided directly to the adder 16. The noise shaper 12 may typically be formed by a digital high-pass filter, which may be based on a delay arrangement or a delay line. When an error signal on the line 26 that is not equal to zero is applied to the noise shaper 12, the noise shaper 12 begins to generate an output value on the line 28, and output values may be generated as a mean value of the amplitudes of the error signal on the line 26.
FIG. 5 illustrates, as an example, a graph of the noise amplitude within the output signal on the line 22 plotted against frequency for an input data signal on the line 14 processed by the circuit 10 of FIG. 4. In the case of the circuit 10 without the noise shaper 12, relatively uniform noise amplitude over all frequencies may be obtained as illustrated by the curve 30 which represents the output signal on the line 22. On the other hand, in the case of the circuit 10 with the noise shaper 12, a noise-shaped output curve 32 is illustrated for the output signal on the line 22, whose signal to noise ratio may be improved in that there is a reduced noise amplitude in a desired range, for example in an audible range 34, together with an increased noise amplitude in a higher, non-audible frequency range. When parameters of the noise shaper 12 are changed, this may cause a corresponding change in the resulting noise spectrum.
If no input data signal is present on the line 20, the signal to noise ratio can be further improved by switching off the noise shaper 12. In particular, switching the noise shaper 12 off or to an inactive state may result in a relatively more robust output signal on the line 22, since the noise shaper 12 in this case no longer generates any noise components. FIG. 6 illustrates an example of a graph with a curve 36 of the reduced noise level of the output signal on the line 22 for the case of the noise shaper 12 being switched off, as compared to a curve 38 of an increased noise level of the output signal on the line 22 when the noise shaper 12 is switched on.
Switching off or inactivation of the noise shaper 12 of the prior art circuit 10 of FIG. 4 may lead to undesirable switching noises, also known as clicks. The switching noises may be caused by the nonlinearity of an impulse function when a signal or a sequence of data from a data sequence is switched off. This type of switching noise typically is independent of the presence or absence of a DC voltage component, thus the switching noise may also occur when the average signal at the output of the noise shaper 12 on the line 28 is zero. The switching noise may typically be due to the low-pass nature of hearing; that is when signal energy is present with a mean value of zero and then is suddenly switched to zero signal energy. This switching process briefly creates frequencies in the overall spectrum, and therefore also in the user frequency band. Thus, with traditional noise shapers 12 such as that of the circuit 10 of FIG. 4, the problem exists of switching off or inactivating the noise shaper 12 in such a way, or generating a specific structure within the noise shaper 12, so that a reduction in noise is achieved.
Most of the known techniques involve noise shapers with a single-bit output. Therefore, the transition to use of a multiple-bit noise shaper may not be possible in certain cases.
U.S. Pat. No. 5,200,750 discloses a circuit in which an additional input signal is provided to the noise shaper to stabilize the noise structure of the shaper. When the structure is stabilized, the noise shaper can be switched off. However, a relatively complex circuit arrangement and procedure are required in this case.
U.S. Pat. No. 5,712,874 discloses a circuit arrangement with a low-pass filter for a first integrator of the noise shaper, to automatically stabilize the noise signal or data sequence of the shaper. An implementation for a multiple-bit noise shaper may not be possible in this case. Further, switching of the low-pass filter may produce a switching noise with an amplitude that is relatively greater than the original switching noise amplitude.
According to a thesis by Thomas H. Hansen, entitled “Muting of Noise-Shaper Quantized Signals”, page 77–94, May 6, 2003, a noise shaper may be halted on the basis of a prediction of the anticipated specific energy. The thesis proposes a method for reducing unwanted in-band transients upon halting of a noise shaper signal. In this method, a detector may be used as a switching mechanism for controlling the instant at which the quantized signal will be set at zero. The switching device for halting the noise shaper may use a model of the noise shaper, on which basis a prediction may be made for the time to switch the noise shaper off.
What is needed is an improved method and device for reducing noise during the switching of a noise shaper.